Recently, a luminous efficiency of an LED has been enhanced and lighting apparatuses using an LED have been mass-produced. In particular, in the past, the trend in the sector of vehicle headlights has employed HID lamps instead of halogen lamps in order to enhance visibility (to enhance brightness). However, with the improvement of the luminous efficiency of the LED, vehicles employing LED headlights are being mass-produced.
FIG. 23 shows a configuration of a conventional vehicle LED lighting device. A DC voltage from a power source E1 that is supplied by interworking with a LOW beam switch is stepped up and down as a voltage for lighting a load by a DC/DC converter 1. A DC voltage as an output voltage from the DC/DC converter 1 is applied to a semiconductor light source 5 to light the semiconductor light source 5. This lighting device lights the semiconductor light source 5 by controlling a constant current, and a control unit 10 is used to perform the control.
A load voltage and a load current of the semiconductor light source 5 are detected by resistors R1 to R3 and inputted to the control unit 10 through a voltage detection circuit 3 and a current detection circuit 4. The control unit 10 averages the load voltage and the load current through averaging units 11 and 12. A comparison calculation unit 15 compares an average current value Ia and a current command value outputted from a ROM in a controller 16 and calculates/outputs a primary side current command value Ic such that the average current value Ia and the current command value become equal. By comparing the primary side current command value Ic and a primary side current detection value Id by a comparator CP, a switching element Q1 of the DC/DC converter 1 is driven.
The switching element Q1 of the DC/DC converter 1 is ON/OFF driven by an output from a flipflop FF as a drive circuit. When the flipflop FF is set by a high frequency ON signal HF, the switching element Q1 is turned on and a gradually increasing current flows through a primary coil of a transformer T1, whereby energy is accumulated in the transformer T1. When the switching element Q1 is an FET, ON resistance thereof is nearly ohmic-resistance, so the primary side current detection value Id can be detected by amplifying a drain voltage by a primary side current detection circuit 2 configured as an OP amplifier or the like. When the primary side current detection value Id reaches the primary side current command value Ic, the output of the comparator CP is inverted and the flipflop FF is reset to turn off the switching element Q1. When the switching element Q1 is turned off, counter electromotive force is generated from a secondary coil due to the accumulated energy in the transformer T1 and the capacitor C1 is charged through a diode D1.
Through the foregoing circuit configuration, the constant current control is performed by PWM-controlling the ON time of the switching element Q1 of the DC/DC converter 1.
In addition to the constant current control, the controller 16 detects an abnormal power or load based on the detection results of the power detection circuit 7, the voltage detection circuit 3 or the current detection circuit 4, and, accordingly, stops the operation of the DC/DC converter 1 and outputs a fault signal.
Further, the control unit 10 is powered by a control power generation unit 6, and a power to the control power generation unit 6 is supplied from the LOW beam switch power source E1. The averaging unit 13 reads and averages power source voltages.
A control flow of the control unit 10 for performing the constant current regulation of the semiconductor light source 5 and determining a fault is shown in FIG. 24. The control unit 10 performs the constant current regulation of the semiconductor light source 5 in steps #04 to #12 and determines a fault of a power and a load in steps #13 to #17. Each step of FIG. 24 will be described hereinafter.
In step #01, a power source is turned on and a reset is released. A reset input is not shown in FIG. 23.
In step #02, the control unit 10 initializes a variable, a flag or the like used in operating.
In step #03, the control unit 10 determines whether or not the LOW beam switch is in an ON state based on an input from the power detection circuit 7. For example, as explained hereinafter, when the power source voltage, which is averaged after being A/D converted and detected by the power detection circuit 7, is greater than 9 V and smaller than 16 V (i.e., 9 V<power source voltage<16 V), the LOW beam switch is determined to be in an ON state. When the LOW beam switch is not determined to be an ON state, a loop of lighting the semiconductor light source 5 after step #04 is not performed.
In step #04, the power source voltage detected through the A/D conversion in the power detection circuit 7 is read out.
In step #05, the averaging unit 13 adds the lately stored detection values to the detection value inputted from the power detection circuit 7 to average the power source voltages. As an example of the averaging, three latest detection values are stored (updated when read out), and when the next value is read, the next value is added to the stored three latest detection values and then the result is divided by four.
In step #06, the voltage detection circuit 3 reads out a load voltage detected through the A/D conversion.
In step #07, the averaging unit 11 adds the previously stored load voltage values to the detected load voltage to thereby obtain an average voltage value Va through the same way as described in step #05.
In step #08, the comparison calculation unit 15 reads out an output current command value from the ROM of the controller 16.
In step #09, the output current detected through A/D conversion at the current detection circuit 4 is inputted to the averaging unit 12.
In step #10, the averaging unit 12 adds the detected output current to the previously stored output current values and an average current value Ia is obtained, as the same way described in step #05.
In step #11, the comparison calculation unit 15 compares the output current command value with the average current value Ia.
In step #12, the comparison calculation unit 15 changes the primary side current command value Ic based on the comparison result.
In step #13, the controller 16 determines whether or not the power source voltage inputted through the averaging unit 13 is normal by checking whether or not the power source voltage is within a predetermined voltage range (from a normal power lower limit to a normal power upper limit). Herein, a range, e.g., from 6 V to 20 V is a normal range. When the power source voltage is abnormal, an operation stop processing (step #15) is performed and the process returns to the LOW beam switch ON determination (step #03).
In step #14, the controller 16 determines whether or not the load voltage inputted through the averaging unit 11 is normal by checking whether or not the load voltage is within a predetermined voltage (from a normal output voltage lower limit to a normal output voltage upper limit). Herein, a range, e.g., from 10 V to 40 V is a normal range. When the load voltage is determined to be normal, the process returns to step #04, and when the load voltage is determined to be abnormal, a load fault signal is outputted (step #16) and permanent stop processing (step #17) is performed.
In step #15, the controller 16 executes operation stop processing (stops the DC/DC converter and clears data within the control unit).
In step #16, the controller 16 outputs a load fault signal in order to inform about the load fault. Specifically, the control unit 10 may inform the fault by outputting a HIGH/LOW signal or by using a communications function or the like.
In step #17, the controller 16 executes permanent stop processing by running an infinite loop of operation stop processing.
Through this control, when the LED as a load has an open/short failure, the fault can be detected by determining whether or not an output voltage is higher than the normal output voltage upper limit and, accordingly, the operation can be stopped.
Patent Document 1 (Japanese Patent Application Publication No. 2006-114279) discloses a technique of reducing an output current value when an output voltage is higher than a normal output voltage upper limit, without having to stop an operation. Further, Patent Document 2 (Japanese Patent Application Publication No. 2006-172819) discloses a technique of reducing an output by using an external interrupt processing to speed up fault detection when a microcomputer is used in controlling.
FIG. 25 shows waveforms of an output voltage and an output current when an output open fault (a situation in which an output power of the lighting device is not provided to a load by a certain reason) occurs. In the related art example, the operation is stopped when an output voltage exceeds a predetermined voltage. For example, regardless whether a load voltage of a connected semiconductor light source is large or not (the load voltage being determined by a forward voltage Vf), the operation is stopped when the output voltage of the lighting device is increased up to a normal output voltage upper limit, which is an upper limit of a load voltage normal range.
However, when an output open fault is generated by a loose contact of bonding of an LED chip or an output connector, it may happen that the contact to the load is open only for an instant and then immediately re-connected (referred to as load chattering hereinafter).
FIG. 26 shows waveforms of an output voltage and an output current when the load chattering occurs. When a semiconductor light source having a high forward voltage Vf is connected, the output voltage is increased up to the normal output voltage upper limit during the load chattering, thereby stopping the operation. And when the load is connected again, the operation starts again. It may be also possible to configure the lighting device not to start the operation after the load is re-connected. However, when a semiconductor light source having a low forward voltage Vf is connected, the output voltage does not reach the normal output voltage upper limit during the load chattering. And when the load is connected again, a voltage much higher than the normal forward voltage Vf is applied to the semiconductor light source, and the output current is stabilized after an over-current flows. The excessive current applies a great load to the semiconductor light source and the lighting device, which, in a worst-case, may lead to inflicting damage on the semiconductor light source or the lighting device.
In addition, when a load is entirely or partially shorted, when the power source voltage is instantly increased, or the like, the output current increases rapidly in a moment, which damages the semiconductor light source or the lighting device. In the foregoing Patent Document 2, a response is quickly made by using an interrupt to a microcomputer or the like. However, since the output voltage or the output current is not increased up to the normal output voltage upper limit or lowered down to the normal output current upper limit, it may not stop the operation. As a result, the semiconductor light source or the lighting device may be damaged.